An anamorphic image is an image that appears distorted until viewed a particular way. Below is Hans Holbein's The Ambassadors with a Memento Mori, a well know example from classical painting (the odd looking blob at the bottom is a skull when viewed from an oblique angle).

The London based artist Jonty Hurwitz has created anamorphic sculptures (above) which reveal their images in reflective cylinders. You can see more of his work at his gallery of recent work.

Via: Colossal Art & Design, which includes a video of his work showing how it appears as you move around the sculpture.